Notes

<trackList> and <trackNum> spelling

Please note that trackList and trackNum elements MUST NOT be written
in all lowercase since XSPF is case-sensitive. These two
are the only elements containing capital letters so just remember trackList
and trackNum and your XSPF will be fine.

Absolute vs. relative URIs in key attibutes

The attributes rel (<meta> and <link>) and application (<extension>)
are by XSPF v1 specification allowed to hold relative URIs. For example this
meta element is legal in XSPF v1:

<meta rel="playCount">14</meta>

While this might look elegant it comes with several drawbacks:

You might easily forget that this string is a URI and include non-URI characters like spaces by mistake (e.g. rel="play count" is not valid)

You lose the namespace concept: nobody will know what or whom this key belongs to or came from

For these reasons it is strongly recommended to use absolute URIs as keys instead:

This is not mentioned in the specification and might
have been ignored by some implementations. Thus for people
implementing XSPF support that means:

XSPF readers must be able to deal with arbitrary element order

XSPF writers should always produce XSPF files with elements in the order appearing in the specification for maximum compatibility.

To save you extracting this information yourself the order
of elements appearing in the XSPF specification is:

playlist

playlist.title

playlist.creator

playlist.annotation

playlist.info

playlist.location

playlist.identifier

playlist.image

playlist.date

playlist.license

playlist.attribution

playlist.attribution.location

playlist.attribution.identifier

playlist.link

playlist.meta

playlist.extension

playlist.trackList

playlist.trackList.track

playlist.trackList.track.location

playlist.trackList.track.identifier

playlist.trackList.track.title

playlist.trackList.track.creator

playlist.trackList.track.annotation

playlist.trackList.track.info

playlist.trackList.track.image

playlist.trackList.track.album

playlist.trackList.track.trackNum

playlist.trackList.track.duration

playlist.trackList.track.link

playlist.trackList.track.meta

playlist.trackList.track.extension

Whitespace

Whitespace handling in XML can be a little difficult to understand at first.
Since XSPF is based on XML, people implementing XSPF readers or writers
do have to know about the basics of whitespace handling in XML.
The good things is we summed it up here to make things easier for you.

Ignore in non-leaf elements

Whitespace in elements only holding child elements (but no text)
must be ignored.

Do X in Extensions

URI RFC used

The XSPF v1 specification mentions RFC 2396 when talking about URIs. As asking on uri@w3.org revealed, we should have referred to STD 66 instead which stands for "latest RFC of URI" (RFC 3986 as of 2007-07-09). What does that mean practically?

Errata

4.1.1.2.12.2 content

This paragraph describes the content of playlist.meta.
It also reads "xspf:playlist elements MAY contain exactly one" which
does not make sense for the content of an element and can be safely ignored.

4.1.1.2.14.1.1.1.12.2 content

This paragraph describes the content of playlist.trackList.track.meta.
It also reads "xspf:playlist elements MAY contain exactly one" which
does not make sense for the content of an element and can be safely ignored.